1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wavefront aberrations, particularly to phase-shifting point diffraction interferometers.
2. State of the Art
Optical metrology is the study of optical measurements. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a mirror or a lens.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. To do so, systems are confronted with the diffraction limit of the light employed to image a pattern provided in a reticle. To meet this challenge, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to x-ray radiation.
Because of the increasing difficulties posed by directly imaging a reticle pattern onto a wafer, it is desirable to use projection optics in lithography systems. Such systems include lenses or other optical elements that reduce the reticle images and project them onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself.
As with all optical elements, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified during the fabrication and/or testing of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram and the resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be "perfect"; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optics through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical system through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly clean interferogram.
To address this problem, "point diffraction interferometers" have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer described in the article H. Medecki et al., "Phase-Shifting Point Diffraction Interferometer", Optics Letters, 21(19), 1526-28 (1996), and in the U.S. Patent Application "Phase-Shifting Point Diffraction Interferometer", Inventor Hector Medecki, Ser. No. 08/808,081, filed Feb. 28, 1997 now U.S. Pat. No. 5,835,217, which are both incorporated herein by reference. Referring to FIG. 1, in this prior art phase-shifting point diffraction interferometer 20, electromagnetic radiation is sent to a pinhole 22. The radiation is then sent through the test optic 24 to a grating 26. The grating 26 produces two beams with a small angular separation. An opaque mask placed near the focal point of the test optic, contains a tiny reference pinhole, and a larger window centered on the respective foci of the two beams. The reference pinhole produces a reference wavefront by diffraction, while the window transmits the test wave without significant spatial filtering or attenuation. In effect, the beam going through the reference pinhole is filtered to remove the aberrations so that this filtered beam can interfere with the test beam that passes through the window without significant spatial filtering. An interference pattern is detected at a detector 30. The light in the interferometer will typically be of a single wavelength. The grating 26 will transmit the zeroth-order beam straight through, but will produce a small angular change to the first-order diffractions. In the image plane, the zeroth-order, and the first-order diffractions will be in different positions, as indicated by the reference pinhole and the test window in the mask 28. Phase-shifting is provided by moving the grating 26 perpendicular to the rulings of the grating. Phase-shifting improves the efficiency and accuracy of the system.
It is desired to have an improved phase-shifting point diffraction interferometer.